87 lines
3.8 KiB
Plaintext
87 lines
3.8 KiB
Plaintext
namespace Eigen {
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/** \eigenManualPage TutorialMapClass Interfacing with raw buffers: the Map class
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This page explains how to work with "raw" C/C++ arrays.
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This can be useful in a variety of contexts, particularly when "importing" vectors and matrices from other libraries into Eigen.
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\eigenAutoToc
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\section TutorialMapIntroduction Introduction
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Occasionally you may have a pre-defined array of numbers that you want to use within Eigen as a vector or matrix. While one option is to make a copy of the data, most commonly you probably want to re-use this memory as an Eigen type. Fortunately, this is very easy with the Map class.
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\section TutorialMapTypes Map types and declaring Map variables
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A Map object has a type defined by its Eigen equivalent:
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\code
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Map<Matrix<typename Scalar, int RowsAtCompileTime, int ColsAtCompileTime> >
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\endcode
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Note that, in this default case, a Map requires just a single template parameter.
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To construct a Map variable, you need two other pieces of information: a pointer to the region of memory defining the array of coefficients, and the desired shape of the matrix or vector. For example, to define a matrix of \c float with sizes determined at compile time, you might do the following:
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\code
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Map<MatrixXf> mf(pf,rows,columns);
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\endcode
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where \c pf is a \c float \c * pointing to the array of memory. A fixed-size read-only vector of integers might be declared as
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\code
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Map<const Vector4i> mi(pi);
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\endcode
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where \c pi is an \c int \c *. In this case the size does not have to be passed to the constructor, because it is already specified by the Matrix/Array type.
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Note that Map does not have a default constructor; you \em must pass a pointer to intialize the object. However, you can work around this requirement (see \ref TutorialMapPlacementNew).
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Map is flexible enough to accomodate a variety of different data representations. There are two other (optional) template parameters:
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\code
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Map<typename MatrixType,
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int MapOptions,
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typename StrideType>
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\endcode
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\li \c MapOptions specifies whether the pointer is \c #Aligned, or \c #Unaligned. The default is \c #Unaligned.
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\li \c StrideType allows you to specify a custom layout for the memory array, using the Stride class. One example would be to specify that the data array is organized in row-major format:
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<table class="example">
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<tr><th>Example:</th><th>Output:</th></tr>
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<tr>
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<td>\include Tutorial_Map_rowmajor.cpp </td>
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<td>\verbinclude Tutorial_Map_rowmajor.out </td>
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</table>
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However, Stride is even more flexible than this; for details, see the documentation for the Map and Stride classes.
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\section TutorialMapUsing Using Map variables
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You can use a Map object just like any other Eigen type:
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<table class="example">
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<tr><th>Example:</th><th>Output:</th></tr>
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<tr>
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<td>\include Tutorial_Map_using.cpp </td>
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<td>\verbinclude Tutorial_Map_using.out </td>
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</table>
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However, when writing functions taking Eigen types, it is important to realize that a Map type is \em not identical to its Dense equivalent. See \ref TopicFunctionTakingEigenTypesMultiarguments for details.
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\section TutorialMapPlacementNew Changing the mapped array
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It is possible to change the array of a Map object after declaration, using the C++ "placement new" syntax:
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<table class="example">
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<tr><th>Example:</th><th>Output:</th></tr>
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<tr>
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<td>\include Map_placement_new.cpp </td>
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<td>\verbinclude Map_placement_new.out </td>
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</table>
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Despite appearances, this does not invoke the memory allocator, because the syntax specifies the location for storing the result.
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This syntax makes it possible to declare a Map object without first knowing the mapped array's location in memory:
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\code
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Map<Matrix3f> A(NULL); // don't try to use this matrix yet!
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VectorXf b(n_matrices);
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for (int i = 0; i < n_matrices; i++)
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{
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new (&A) Map<Matrix3f>(get_matrix_pointer(i));
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b(i) = A.trace();
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}
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\endcode
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*/
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}
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